A coating is often applied to metal substrates, especially metal substrates that contain iron such as steel, prior to the application of a protective or decorative coating. The coating minimizes the amount of corrosion to the metal substrate, if and when, the metal substrate is exposed to moisture and oxygen. Many of the present coating compositions are based on metal phosphates, and rely on a chrome-containing rinse. The metal phosphates and chrome rinse solutions produce waste streams that are detrimental to the environment. As a result, there is the ever-increasing cost associated with their disposal.
Coating compositions can be applied without chrome rinse solutions. For example, U.S. Pat. No. 3,966,502 discloses post-treating phosphated metals with zirconium-containing rinse solutions. However, this application process is only suitable for use over a limited number of metal substrates, and the generation of metal phosphate waste streams is not alleviated.
U.S. Pat. No. 5,534,082 to Dollman et al. and U.S. Pat. Nos. 5,281,282 and 5,356,490 to Dolan et al. describe non-chrome coating compositions containing a fluoroacid such as fluorotitanic acid, dispersed solids such as silica, and a water-soluble polymer such as an acrylic acid polymer and/or a polymer with hydroxyl functionality. The coatings compositions enhance the corrosion resistance of steel and galvanized steel substrates. Also, the pH of these compositions are strongly acidic, and range from 0 to 4, preferably from 0 to 1.
U.S. Pat. No. 5,938,861 to Inoue et al. describes forming a coating on metal substrates, except aluminum. The coating composition includes an oxidative compound such as nitric acid or hydrogen peroxide, silicate or silicon dioxide particles, and a metal cation, oxymetal anion, or fluorometallate anion of Ti, Zr, Ce, Sr, V, W, amd Mo.
U.S. Pat. No. 5,859,106 to Jones et al. describes a non-chrome coating composition said to contain a cross-linked polymer system and a fluoroacid such as fluorozirconic acid or fluorotitanic acid. The polymer system includes a copolymer with acrylic and hydroxyl functionality or the reaction product of an acrylic polymer and a polymer with hydroxyl functionality. U.S. Pat. No. 5,905,105 to Jones et al. describes a non-chrome coating composition that includes the coating composition described in U.S. Pat. No. 5,859,106 and dispersed silica. The dispersed silica is present in the compositions in an amount from 40% to 70% by weight, calculated on a dry weight basis.
There is an interest to develop coating compositions and methods of applying such compositions without producing metal phosphate and chrome waste solutions. It is also preferred, that these coating compositions be effective in minimizing corrosion in a variety of metal substrates because many objects of commercial interest contain more than one type of metal substrate. For example, the automobile industry often relies on metal components that contain more than one type of metal substrate. The use of a coating composition effective for more than one metal substrate whould provide a more streamlined manufacturing process.